A power semiconductor device is known in which an electric power is supplied to a load by using an output transistor. For example, the power semiconductor device for a car is used to drive a load such as a lamp, a solenoid and the like. In the car, when a covering of a harness connecting the load is stripped and the harness is consequently brought into contact with a body (GND) of the car, this leads to a load short circuit situation, and an excessive electric power is applied to the output transistor. To protect the output transistor from having trouble caused by the foregoing load short circuit situation, the power semiconductor device includes an overcurrent protection circuit. One of the best known circuits as the overcurrent protection circuit is a current limit circuit. Under the overcurrent situation, the current limit circuit controls a gate voltage of the output transistor to carry out a feedback control such that a current flowing through the output transistor is kept at a constant value. As an example of the foregoing current limit circuit, a patent literature 1 (J P Heisei 11-285146A) discloses a protection circuit for a voltage-driven power semiconductor device.
The protection circuit for the voltage-driven power semiconductor device in the patent literature 1 includes an output current monitor means, an overcurrent protection means and an output current limit means. The output current monitor means monitors an output current of the voltage-driven power semiconductor device. The overcurrent protection means tries to protect the output current from exceeding a predetermined value by adjusting a gate potential of the voltage-driven power semiconductor device, based on a current value monitored by the output current monitor means, when the output current of the voltage-driven power semiconductor device exceeds the predetermined value. The output current limit means limits the output current based on a voltage that is applied between both ends of the voltage-driven power semiconductor device. In this current limit circuit, mainly, a current limit value is changed like stairs based on a voltage between a drain and a source of the output transistor (FIG. 2 of the patent literature 1). The current limit value may be linearly changed in a certain range (FIG. 6 and FIG. 12 of the patent literature 1).